Who is Cynthia Knox from The Widening Gyre and what is their importance?

Cynthia Knox is one of the participants in Broz's orgies. She is described as beautiful and classy, but her experiences with Broz leave her feeling dirty. Cynthia is unhappy and has chosen to self-medicate with drugs and casual sex. Spenser urges her to find another way..... and she agrees to help him with his investigation in any way she can, so long as he promises to guard her reputation.